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From: Bosko Milekic <bmilekic@FreeBSD.org>
Date: Thu, 10 Jun 2004 00:04:27 +0000 (UTC)
To: src-committers@FreeBSD.org, cvs-src@FreeBSD.org,
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Subject: cvs commit: src/sys/kern uipc_mbuf.c
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bmilekic    2004-06-10 00:04:27 UTC

  FreeBSD src repository

  Modified files:
    sys/kern             uipc_mbuf.c 
  Log:
  Plug a race where upon free this scenario could occur:
  
  (time grows downward)
  thread 1         thread 2
  ------------|------------
  dec ref_cnt |
              | dec ref_cnt  <-- ref_cnt now zero
  cmpset      |
  free all    |
  return      |
              |
  alloc again,|
  reuse prev  |
  ref_cnt     |
              | cmpset, read
              | already freed
              | ref_cnt
  ------------|------------
  
  This should fix that by performing only a single
  atomic test-and-set that will serve to decrement
  the ref_cnt, only if it hasn't changed since the
  earlier read, otherwise it'll loop and re-read.
  This forces ordering of decrements so that truly
  the thread which did the LAST decrement is the
  one that frees.
  
  This is how atomic-instruction-based refcnting
  should probably be handled.
  
  Submitted by: Julian Elischer
  
  Revision  Changes    Path
  1.132     +30 -15    src/sys/kern/uipc_mbuf.c